Electromagnetic radiation with wavelengths in the millimeter and/or micrometer range is reflected by metallic materials and a number of non-metallic materials such as ceramics or specific plastic materials. Air and many other materials such as, for example, clothing materials are transparent for such radiation. The radiation is also non-ionizing, and therefore millimeter waves are particularly suited for contactless detection of, for example, objects which may be hidden under a person's clothing. Millimeter-wave scanning systems can complement or replace other detection systems such as metal detection systems employed for security gates at airports, football stadiums, etc.
The term ‘millimeter waves’ as used herein may also cover micrometer waves, i.e. electromagnetic radiation in the Gigahertz (GHz) and Terahertz-range (THz). Corresponding scanning systems are operable to reconstruct an image of a person and/or object and may comprise for said purpose an array formed of a plurality of transmitting antennas and an array formed of a plurality of receiving antennas, wherein the arrays may be separate or may overlap. For example, an overlapping aperture may result from arranging transmitter and receiver antennas at a common panel.
One object to be scanned is then illuminated with millimeter waves transmitted from each of the transmitting antennas; for example, the transmitting antennas may be controlled to transmit radiation one after the other. In a multistatic mode, all available receiving antennas may be operable to detect radiation, which is assumed to comprise radiation of the currently active transmitting antenna reflected by the object to be scanned. Often, the sequence of transmission antennas is traversed multiple times with different frequencies according to a given frequency vector.
The measurement data may represent amplitude and phase information of the detected received radiation, even if not stored explicitly in such format, but in another complex-type format as known for signal processing. Image reconstruction may be performed by performing complex operations on the complex-valued measurement data wherein a final object image may be obtained by pixel-wise calculating magnitude values from complex number representations.
Systems for scanning objects such as persons have to tolerate object movements or motions to some extent. For example, there is a general requirement for convenient walk-through scanning systems. However, image blurring may occur if during the measurements a person or other imaged object moves a distance of the order of the scanning wavelength. For currently available systems, movements such as typical for walk-through scanning can lead to serious image blurring, i.e. ideally a person to be scanned should stand still during scanning. Optimizations are therefore required.
A. Schiessl, S. S. Ahmed, and L.-P. Schmidt, “Motion Effects in Multistatic Millimeter-Wave Imaging Systems,” in Proc. SPIE 8900, Millimeter Wave and Terahertz Sensors and Technology VI, 2013 describe and discuss the problem of short measurement times which should allow scanning people in uninterrupted, normal comfortable movement. Several measures are recommended. For example, it is proposed to replace system with mechanical focusing by fully electronic systems without moving parts, but with digital beamforming for focusing transmitted and/or receiving antennas. Forced application of multistatic configurations allows operating many receiving antennas in parallel which can also contribute to reducing measurement times as compared to monostotic configurations. High-performance electronic components and subsystems should be employed, however, such components are available at high costs only and/or are not available to date. Still further approaches are required.
In the millimeter-wave image reconstruction field, there is a need for an approach for minimizing image blurring in case of a moving person or object to be scanned, which approach should preferably be appropriate for being implemented cost-efficiently, for example based on existing hardware.